Some kinds of thin workpieces (e.g., a wafer on which an IC device is to be formed) must be polished with a high precision to have a flat surface. One or both surfaces of these workpieces are polished by polishing machines. A machine which can polish both surfaces of such a workpiece is known. As shown in FIG. 1, this machine comprises an upper polishing plate 2 and a lower polishing plate 4 located below the plate 2. Two pieces 6 and 8 of a polishing pad are bonded to the opposing surfaces of the polishing plates 2 and 4. The machine further comprises a tank 10 containing slurry. The slurry is supplied from the tank 10 to the polishing pad 6 through a hose 11 and a passage hole 12 cut in the plate 2 and is used as an abrasive. The passage 12 communicates with a hole 14 which is cut in a piece of the polishing pad 6. Two plugs 16 and 18 are secured to the ends of the hose 11 and detachably attached to the tank 10 and the upper polishing plate 2, respectively.
The polishing pad 6 bonded to the lower surface of the upper polishing plate 2, which consists of three plates 20, 22 and 24, can be replaced by a new one in the following steps:
1. First, the lowest plate 24 is removed from the plates 20 and 22, which are fastened together.
2. The plate 24 is turned upside down, so that the pad 6 faces up. The pad 6 is peeled off the plate 24, and a new emery cloth is bonded to the plate 24.
3. That portion of the new polishing pad 6 which is directly above the passage 12 of in the plate 24 is detected by touching the pad with one's fingers or piercing the pad with a needle. The hole 14 is then cut in the detected portion of the pad 6 with a rod-shaped cutter or a razor.
4. The plate 24 is again turned over and placed on the lower polishing plate 4.
5. The plate 24 is moved until the passage 12 is axially aligned with those of the plates 20 and 22. The plate 24 is fastened to the plates 20 and 22 with bolts 26 (only one is shown in FIG. 1).
Much time is needed to remove the plate 24 from the plates 20 and 22 and fasten it thereto. Since the plate 24 is too heavy for one person to hold, two or more people must cooperate to replace the polishing pad 6 with a new piece. Further, every time the pad 6 is changed, the plate 24 must be moved. There is the risk that, if the plate 24 hits something, its flat surface would be impaired. If the plate 24 becomes less flat, the workpiece polished by the apparatus cannot be as flat as desired.
When the hole 14 is cut in the polishing pad 6, small scraps of cloth inevitably fall into the passage 12 in the plat 24. Unless these scraps are removed from the passage 12, slurry cannot flow smoothly through the hole 12.